Oxygen transport through high-purity, large-grain Ag
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S. N. Sankaran Analytical Services and Materials, Inc., Hampton, Virginia 23666
G.B. Hoflund and M. R. Davidson University ofFlorida, Gainesville, Florida 32611
(Received 12February 1988; accepted 23 June 1988) The permeation of oxygen through high-purity, large-grain Ag membranes has been studied over the temperature range of 400-800 °C. The permeability was found to be linear and repeatable, but the magnitude was 3.2 times smaller than that determined by past research. This factor may be due to negligible grain boundary diffusion that exists in this work. Auger electron spectroscopy (AES) does, however, suggest the importance of grain boundaries since intragranular oxygen was virtually undetectable and since AES line scans show substantial oxygen signals around the grain boundaries. The diffusivity measurements were found to exhibit two distinct linear regions, one above and one below a critical temperature of 630 °C. The hightemperature data have an activation energy (11.1 kcal mol~') similar to that reported by others, but the low-temperature data have a comparatively larger activation energy (15.3 kcal mol ~~'). Vacuum desorption of the oxygen-saturated Ag was found to occur at the critical temperature of 630 °C, which is consistent with the increased mobility of oxygen atoms in the higher temperature regime. The higher activation energy observed in the lower temperature regime is probably due to the higher efficiency of traps.
I. INTRODUCTION Past measurements of the diffusivity of oxygen in Ag have, in all cases, ignored the effects of grain boundary density and material purity. '~6 Since both are important concerns in determining the actual transport mechanism, it is essential that the effects of each be studied. If grain boundary diffusion is a significant part of the transport mechanism for the diffusion of oxygen in Ag, then grain boundary density would have a major effect. Experiments with larger grain size would reduce the number of transport paths and would provide smaller permeation rates compared with past research (which most likely had small grains and therefore higher grain boundary densities). Furthermore, impurities (both species and amounts) could cause oxygen trapping effects that would result in diffusivity variations. Experiments with high-purity Ag would minimize such effects. This paper presents results of permeation measurements of oxygen through high-purity, large-grain Ag membranes conducted under ultra-high vacuum (UHV) conditions. Auger electron spectroscopy (AES) results of oxygen charged Ag are also presented. II. PERMEATION ANALYSIS The permeation of planar membranes is generally described in Crank.7 If a diffusion coefficient D independent of concentration C is assumed, then 1378
(1)
dt
where x is the variable distance through the membrane and t is the time. The boundary conditions applied to this equation can be determined from the permeation process, which includes (1) adsorption of the oxygen molecules on the high-pressure side; (2) dissociation of the molecules to atoms
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